The invention relates to a projection television display tube comprising an evacuated envelope having a display window provided on its inside with a display screen and a transparent second window which is disposed in front of the outside of the display window. Transparent coolant flows through the space between the display window and the second window, said coolant conveying the heat taken up at the display window through a cooling member to the atmosphere.
A display tube of this type is disclosed in DE-OS 30 21 431, to which U.S. Pat. No. 4,529,905 corresponds. A field is written with the aid of an electron beam on a display screen having a phosphor coating or a pattern of different phosphors. Due to the electron bombardment, the temperature of the phosphor increases so that the light output of the display screen decreases ("thermal quenching"). This phenomenon occurs particularly in display tubes for projection television in which the display screen is scanned by electron beams having high beam currents to obtain the required high luminous fluxes. The temperature of the display window increases and brings about a temperature gradient which causes a mechanical stress in the display window. At a high electron beam current and consequently a high thermal load this may lead to breakage of the display window. To reduce this mechanical stress in the display window due to variations in temperature ("thermal stress") and to obviate the decrease in light output the display window and the display screen are cooled. In a first described embodiment the coolant-filled space between the display window and the second window is surrounded on the upper, lower and lateral sides with a metal cooling member serving as a spacer and operating as a heat radiator. Due to the increase in temperature of the display window the coolant heated by the display window moves upwards along the display window and downwards along the second window so that the heat is also dissipated from the centre of the display window through the cooling member. At a low load, for example, less than 5 W, the heat is mainly dissipated by conduction to the second window. At a higher load the above-described flow of coolant occurs with little additional cooling.
Cathode ray tubes of up to approximately 40 W beam current capacity can be operated continuously with such a closed cooling system. A serious drawback of the known picture tube is, however, that there are no measures for operating the tube for a specific period at a value exceeding the permitted load of approximately 40 W. In fact, the thermal dynamic range of the known picture tube is essentially only defined by the available heat capacities of the coolant, cooling member and tube member. The result is a rapid temperature increase of the coolant and the display window exceeding the permitted temperature.
U.S. Pat. No. 4,529,905 also describes an embodiment in which the coolant is subjected to a cooling outside the space. To this end the coolant is circulated from the top of the space through pipe or tubing through a cooling chamber to the bottom of the space as a result of temperature differences in the coolant. With such an open cooling system it is possible to conduct up to 100 W and more beam current capacity away from the display window, but these open systems are technically very cumbersome since they require an external coolant circulation and a heat exchange separated from the display tubes. Due to the high manufacturing costs the open systems are not suitable for colour television projection apparatus in the domestic range. A further drawback of such a tube is that to replace the tube in a projector, the coolant must be removed and the tubing or pipe must be detached from the display tube.